One morning a few months ago, I was standing on a Central line platform at Bond Street tube station. As a train came in, a man dashed up holding the hand of a young boy. "Just in time, dad!" exclaimed the son, who seemed touchingly unaware that there is a train every two minutes on the Central in peak hours.

When I was a boy visiting London from Yorkshire, it was the train frequency that impressed me. Where did they all come from? The underground was like a magician, producing a constant succession of rabbits from a hat. Above-ground railways seemed so niggardly by comparison, and the gateway to them was the timetable, which reminded me of the logarithm tables which I was at the time condemned to grapple with. I began to wonder why all railways couldn't be like the London underground, and in this week marking the 150th anniversary of the system, I wonder still.

Whereas Britain's above-ground railways were built by men who wanted to make money, the underground was conceived by an altruist: the lawyer and social reformer Charles Pearson, who envisaged a railway that would allow the working classes inhabiting the rookeries of Farringdon and Clerkenwell to live in the pretty villages west of Paddington.

The line – the Metropolitan – was built when Pearson teamed up with men who wanted to make money, but the underground has always been "the people's" railway. Consider Leslie Green, architect of many of the Edwardian tube stations. He gave each a different, and beautiful, tiling scheme so illiterate Londoners would recognise their home stations.

That ethos continues (against all the odds, you feel elated in the new tube ticket hall of King's Cross St Pancras), and this is possibly because the underground has been publicly owned since 1933. Yes, there was the farcical interlude of the Blairite private public partnership, when an attempt was made to lure private finance by putting the railway into the hands of accountants. But essentially the penny has dropped that: (a) the underground is not going to make a profit; and (b) life would be impossible without it.

So there have been no line closures, apart from a bit of trimming at the edges. Whereas our national railway lost the battle with the road lobby between the 60s and 80s, the underground saw it off, largely thanks to Ken Livingstone, who also introduced the network zoning that led to the automatic ticket barriers, leading in turn to the Oyster card. (Pre-Livingstone, the fares were as dismayingly numerous as on our privatised national railway.)

Whereas on much of the "big" railway, human warmth has been replaced by the stare of the CCTV camera, every underground station is manned, and a costermonger-ish bonhomie is suggested by the shouts of "Mind the doors", or "Move down inside the cars" ... And by and large we do move down, and we do "Stand on the right", in return for which the public address calls us "Ladies and Gentlemen".

The underground is embedded in the folklore of London. When I wrote a weekly column about it, a man sent me a letter describing the "notches on the Travelcard" denoting the true Londoner. To qualify, he or she must have left an umbrella on the tube, drunkenly run up the down escalator, missed the last train, fallen asleep on the last train and woken up at the terminus, and so on.

The tube user is spared many of the irritations of modern train travel. It's unlikely that your neighbour will start watching a DVD of The Bourne Identity through leaky earphones. Mobile phones can't be used on the deep-level lines; there are no reservations, so you can't be sitting in someone else's seat. There can be "leaves on the line", I admit, and frozen points. 55% of the underground is overground, as people like me love to point out; and given the slightest encouragement we will go into the full repertoire of the "amazing facts" the underground so effortlessly generates. (Did you know that one million gallons of water are pumped away from beneath Victoria underground station every day?)

To some of my fellow rail fans the underground is a special, remedial case, mere plumbing for London. But think of London and you think of the tube map; the underground is the one common denominator, cause and perpetuator of the great expansion of the city, the Iron Horse run rampant.